Drop pixbuf support in IconHelper and ImageDefinition

Drop pixbuf support in IconHelper and ImageDefinition

These are no longer used, instead we always covert to surface as
early as possible and drop the pixbuf.

This means we never store both the pixbuf and the surface at
for any longer time, which is wasteful. Also, its one step further
to drop GdkPixbufs from generic use in our APIs.